The smooth development of the Internet, which has expanded at a rapid pace, is being sustained by an increase in the capacity of communications by virtue of optical fiber. The scope of application of optical fiber communication networks has broadened to include not only trunk line systems but also access systems and subscriber systems. Inexpensive semiconductor lasers capable of withstanding high-temperature operation are in demand particularly in FFTH (Fiber-To-The-Home), a system in which optical fiber is connected to the home of each subscriber. In semiconductor lasers for optical subscriber systems, achieving a low oscillation threshold current, a low driving current and a low driving voltage is essential for the purpose of reducing power consumption, and a BH (Buried Hetero) structure generally is employed. In particular, a PBH (Planar Buried Hetero) laser in which a pnpn thyristor layer is adopted as a current narrowing/blocking structure and both sides of the light-emitting portion are buried is in wide use in order to reduce leakage current (reactive current).
As an example, reference will be had to the drawings to describe the structure and method of manufacturing a buried semiconductor laser on an InP substrate according to the conventional art, which is set forth in Patent Document 1 and Non-Patent Document 2.
FIGS. 10A-10D is a diagram useful in describing a method of manufacturing a conventional semiconductor laser, and FIG. 11 is a perspective view of the semiconductor laser.
First, an active layer 22 comprising InGaAsP or the like and a p-type InP cladding layer 23a are grown epitaxially on an n-type InP substrate 21 successively (FIG. 10A). Next, a dielectric mask 24 comprising SiO2 or the like is formed in the shape of a stripe on the p-type InP cladding layer 23a and, using this as a mask, the p-type InP cladding layer 23a, active layer 22 and n-type InP substrate 21 are etched into a mesa stripe (FIG. 10B). A current narrowing/blocking structure comprising a p-type InP blocking layer (buried layer) 25 and n-type InP blocking layer 26 is then grown burying the sides of the mesa stripe (FIG. 10C). After the dielectric mask 24 is removed, a p-type InP over-cladding layer 23b and a p-type InGaAs contact layer 27 are grown epitaxially (FIG. 10D).
By subjecting the wafer (FIG. 10D) that has thus undergone epitaxial crystal growth to an electrode formation process for a p-side electrode 28 and n-side electrode 29, the semiconductor laser of FIG. 11 is obtained.
In general, Zn on the order of 1×1018 cm−3 is used as the dopant (acceptor) in the p-type InP blocking layer 25 and p-type InP over-cladding layer 23b, and Si on the order of 1×1018 cm−3 is used as the dopant (donor) in the n-type InP blocking layer 26.
It should be noted that Patent Document 2 discloses a method of manufacturing a semiconductor laser in which a rise in resistance of a cladding layer due to diffusion of hydrogen is prevented while the introduction of a Group-V defect in the cladding layer and active layer is prevented by carrying out a temperature lowering process, which follows the formation of a heterostructure, in an atmosphere that includes a hydrogen-containing compound serving as a Group-V raw material and carrying out a temperature lowering process, which follows the formation of a p-type contact layer, in an atmosphere that does not include a hydrogen-containing compound serving as a Group-V raw material.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-2007-103581A (FIG. 1)
[Patent Document 2]
Japanese Patent Kokai Publication No. JP-P2002-26458A
[Non-Patent Document 1]
Ikegami, Tsuchiya, Mikami, “Semiconductor Photonic Device Engineering”, Corona Publishing Co., Ltd., Jan. 10, 1995, pp. 202-203